The effects of copper and magnesium on the precipitation characteristics of Al-Li-Mg, Al-Li- Cu, and Al-Li-Cu-Mg alloys have been investigated during isochronal and isothermal ageing. In AI-Li-Mg alloys, increasing the magnesium concentration results in stimulation of δ'precipitation by a shift of the α/δ'solvus boundary to higher temperatures. It was shown that for each wt%Mg present in the alloy the α/δ'solvus boundary shifts by 7.0°C. In Al-Li-Cu alloys the concentration of copper has no effect on the position of the α/δ'solvus boundary. The significant stimulation of δ' observed in Al-Li-Cu alloys was shown to be due to the formation of GPCu zones that act as heterogeneous nucleation centres. TEM analysis showed that this heterogeneous nucleation produced composite precipitates consisting of an inner plate of GPI zone and an outer cylindrical shell of δ'. At high copper concentrations (Cu>2.0%) and long ageing times at 150°C, significant retardation of δ' precipitation takes place due to precipitation of the equilibrium T1 and T2 phases. The mechanisms by which copper and magnesium affect the precipitation characteristics of Al- Li-Cu-Mg alloys are different than those operating in the ternary AI-Li-Mg alloys and Al-Li-Cu alloys. In 1.7Lil. 2CuXMg alloys, increasing the magnesium concentration beyond 1.2% causes significant stimulation of δ'precipitation through the formation of Li-Cu-Mg clusters (mechanism referred to as CL δ') that are capable of rapidly developing into δ'. It is proposed that in 1.7Lil. 2CuXMg alloys the initial 1.2%Mg added is consumed in the formation of GPB zones that have very little effect on δ' precipitation. As the magnesium concentration increases to levels higher than 1.2%, the magnesium is free in the matrix to gather both copper and lithium thus forming Li-Cu-Mg clusters which are extremely effective at nucleating δ' In 1.7Li1.2MgXCu alloys the mechanisms by which stimulation of δ' precipitation takes place are again by formation of Li-Cu-Mg clusters (CL δ'), and by nucleation on GPB zones (mechanism referred to as GP δ'). During ageing at 70 and 100°C, and for copper concentrations in the range 0-1.2%, the dominant precipitation mechanism is GP δ'. For higher copper concentrations (1.2